Biochar in Composting

Biochar in Composting

Reduced nitrogen loss, increased microbial activity, shorter time till maturity and significantly less odor have all been observed when biochar was used as a compost amendment (Steiner et al., 2010, Khan et al., 2014, Fischer, Glaser, 2012).


This is it, perhaps the most efficient place to start with biochar. Biochar’s use in the composting process is mutually beneficial. The composting process can be improved when amended with biochar.  The biochar is not diminished during composting, in fact the opposite is true, the biochar becomes improved as well.  And the garden where it lands gains the benefit of both.  Here are some tips on how to get it right.

Potential Benefits

Reduced nitrogen loss is the most commonly valued benefit.  Where biochar is used in composting, it is very common to observe that less nitrogen is lost to the environment (more of it stays in the compost).  This can result in a better finished product whose nutrient profile better represents the plant material it was created from, and also has significant environmental benefit when greenhouse gas emissions from composting are reduced.compost with biochar

Compost has been observed to mature faster with biochar.  This is only from anecdotal evidence so far, but has been noted often.  It makes sense in that biochar has often been valued for its ability to support microbial activity, acting as a refuge, and as a catalyst.

Aeration and moisture buffering.  Biochar is a light fluffy material that can hold lots of water.  Helping buffer moisture and aeration in compost piles with biochar is a benefit that has been observed but not well studied.  I wouldn’t count on the biochar working miracles, you’ll still have to turn your piles, add water, and monitor as usual, but from initial evidence, it seems like adding a little bit of light fluffy biochar might make your job a tad bit easier.

Microbial dynamics.  Nothing in agriculture is more simple and straightforward than microbial dynamics (that was a sarcastic joke).  Two very common problems in composting stem from 1.) nutrient imbalances (too much or not enough) and 2.) lack of oxygen (anaerobic conditions).

  1. Biochar is an inert material that acts as a filter (think charcoal filter), it can hold onto nutrients and serve as a bank for those nutrients.  Nutrients held on the surface of biochar are less reactive, but largely still available (the microbes can still access them when desired).  This can help in buffering the fluctuations in nutrient balance.
  2. Biochar can be a light a fluffy material similar to perlite but with incredible porosity on a microscopic level.  My own personal experience is that biochar promotes aerobic conditions, but to date I am not aware of any peer reviewed research articles that I could point you to as scientific evidence that this is true.  The results that I have observed, and others as well, suggest that this may help explain the results we see.

biochar unloading at a compost yardApplication Rates

Biochar is not available carbon, but the carbon:nitrogen balance of the pile still serves as a guide towards the right application rate.

  • If nitrogen is a limiting factor, only a small amount of biochar is recommended, 5% or less.
  • A well balanced compost can benefit from application rates of biochar at 5% to 10% by volume.
  • Fresh manure and other high nitrogen materials benefit from higher application rates of biochar; 10% to 20% by volume.

The texture of the compost matters as well. Biochar, when added to a compost pile, can improve aeration and also reduce moisture loss – thus improving microbial activity and possibly maturation rate (Khan, Naser, 2014). Rates for biochar application to compost to improve aeration depend on the texture of the other materials; add biochar till it feels right.

pH matters

The production of biochar results in some ash being formed. Ash is alkaline. While the carbonized plant matter (the black stuff) is essentially neutral in pH, the ash portions are not. The Blacklite pure biochar material we provide has a pH of about 10, but a liming value (CaCO equivalent) of 15% or less. In other words, it has a high pH but does not pack a heavy punch.  The Blacklite Mix #6 has a lower pH, usually about 8.  For some uses the alkalinity is a benefit, for others a burden, use accordingly.